Computer systems receive various forms of input and generate various forms of output, of which sound, text, and graphics are examples. Text and graphics may be displayed on a display surface, such as a monitor. Some computing devices may integrate input and output functionality within a common physical structure—for instance, by imparting input capability to an interactive display surface. Touch-sensitive display screens are examples of this approach.
Touch-sensitive devices may utilize any of a number of possible touch sensing mechanisms, including, but not limited to, optical, resistive, and capacitive mechanisms. In any of these forms, a touch-sensitive device may allow the detection and utilization of input that is based on a position of one or more physical objects on a touch-sensitive display surface. For example, the acts of placing an object on a surface, lifting it off a surface, and moving the object from one location on a surface to another each may be a form of input.